# std::atomic_ref<T>::operator+=,-=

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Provided only when `T` is an arithmetic type other than cv bool or a pointer-to-object type |  |
value_type operator+=( difference_type arg ) const noexcept; | (1) | (constexpr since C++26)
value_type operator-=( difference_type arg ) const noexcept; | (2) | (constexpr since C++26)

Atomically replaces the current value referenced by *`ptr` with the result of computation involving the previous value and arg. These operations are read-modify-write operations.

1) operator+= performs atomic addition. Equivalent to return fetch_add(arg) + arg;.
2) operator-= performs atomic subtraction. Equivalent to return fetch_sub(arg) - arg;.

- For signed integral types, arithmetic is defined to use two’s complement representation. There are no undefined results.
- For floating-point types, the floating-point environment in effect may be different from the calling thread's floating-point environment. The operation need not be conform to the corresponding std::numeric_limits traits but is encouraged to do so. If the result is not a representable value for its type, the result is unspecified but the operation otherwise has no undefined behavior.
- For pointer types, the result may be an undefined address, but the operation otherwise has no undefined behavior.
  - If std::remove_pointer_t<T> is not a complete object type, the program is ill-formed.

These overloads participate in overload resolution only if std::is_const_v<T> is false.

### Parameters

arg | - | the argument for the arithmetic operation

### Return value

The resulting value (that is, the result of applying the corresponding binary operator to the value referenced by *`ptr` immediately preceding the effects of the corresponding member function).

### Notes

Unlike most compound assignment operators, the compound assignment operators for `atomic_ref` return a copy of the stored value instead of a reference to arg.

### Example

| This section is incomplete Reason: no example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3508 (P3323R1) | C++20 | compound assignment operators were meaningless for const T | constrained to accept only non-const `T`

### See also

fetch_add | atomically adds the argument to the value stored in the referenced object and obtains the value held previously (public member function)
fetch_sub | atomically subtracts the argument from the value stored in the referenced object and obtains the value held previously (public member function)
operator++operator++(int)operator--operator--(int) | atomically increments or decrements the referenced object by one (public member function)
operator&=operator|=operator^= | atomically performs bitwise AND, OR, XOR with the referenced value (public member function)

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